Survival in the keenly competitive magnet wire marketplace requires large scale production of high quality product at low cost. There is an intense and continual effort in the industry to develop cost reducing process improvements to provide competitive advantage.
The typical process for producing magnet wire is a continuous three-step process.
In the first step, bare wire is annealed. Wire of a desired diameter is drawn from a supply through an annealing oven in order to soften it and increase its flexibility as required in the subsequent process steps.
In the second step, the bare wire exiting the annealing oven is coated with a layer of insulating enamel by, for example, passing through an open slip containing the enamel. The enamel used for insulating magnet wire is typically a polymer solution, such as a polyamide, a polyurethane, a polyester, or a polyimide, dissolved in an organic solvent or mixture of organic solvents, such as phenol and cresol. The coated wire passes through a die having a passage of a dimension to allow only the wire and a layer of insulating enamel adhering to the wire to pass through to form a coated wire.
In the third step, the enamel on the coated wire is dried and cured by heating. A vertical oven is typically used. A temperature gradient is maintained along the path of the coated wire being drawn through the oven. At the bottom where the coated wire enters, the temperature is maintained to allow the gradual evaporation of the organic solvents from the enamel. At the top end of the oven where the coated wire exits, a relatively higher temperature is maintained to cure the enamel. The wire exits the oven with a coating of cured enamel.
The insulated wire typically undergoes a series of coating steps as the insulation layer formed by one coating step is too thin for most applications. The insulated wire is taken from the oven and drawn through a second coating step. A second layer of insulating enamel adheres to the wire, which is then passed through a second die which has a passage of larger dimension than the first die, and drawn through the oven for drying and curing of the second layer. It should be noted that the second die functions in the same manner as the first die, and differs only in that its passage is of a slightly larger dimension than that of the first die. This allows for the thickness of the coating of the insulation to be increased by the second coating step.
This method of adding coatings to the wire may be repeated any desired number of times and requires only that successive dies have passages with progressively increasing dimensions to assure the formation of a progressively increasing thickness of insulation on the wire. The successive layers of enamel may be of different composition in order to take advantage of the unique properties of each of the enamel coatings.
The dies used in the coating step may be retained in a structure known as a die bar. Typically, the die bar is a rectangular bar having suitably formed holes within which individual dies may be secured. A bare wire from a feed spool or an insulated wire returning from the oven exit to be recoated is fed to a feed sheave situated below the die bar, passes upward through the die and the oven and is fed to a return sheave situated above the exit of the oven. The return sheave acts to receive the insulated wire exiting the oven and either return it to the feed sheave for subsequent recoating or feed the wire to a take-up spool where the fully insulated or product wire is wound.
The production of insulated magnet wire usually entails the simultaneous production of a plurality of separate wires each undergoing multiple passes through the process path. In this way several separate wires may be simultaneously produced to effect a savings in production time and energy. This method requires the use of a plurality of sheaves, and a plurality of dies, and perhaps a plurality of die bars.
While the benefits with regard to efficiency and energy savings of a multiple wire, multiple pass type operation should be apparent, the method is not without disadvantages. Adjusting the alignment of the die bars and guide sheaves as, for example, when it is desired to change dies, is a time-consuming process. Changing enamel materials is also a time-consuming process. The enamel supply system and the dies must be flushed with solvent to remove traces of the previous enamel. The need to dispose of flushing solvent further increases the cost of this technique. Further, if conditions require an adjustment of process parameters affecting one wire, the entire line must typically be shut down, greatly multiplying the cost of the adjustment.
What is needed in the art is a magnet wire coating apparatus and method which overcomes the above problems.